No major damage was reported, but authorities in Puerto Rico said an elderly woman died in an accident while preparing for the storm.

U.S. forecasters said Isaac was likely to approach Hispaniola, the island shared by the Dominican Republic and Haiti, as a hurricane late Thursday or early Friday after intensifying over the warm waters of the Caribbean. It was predicted to move on to Cuba as a tropical storm, then perhaps head by Monday to Florida, where the Republicans will be gathering to nominate Mitt Romney for the presidency.

In the eastern Caribbean, many seafront bars and restaurants remained open Wednesday night as lightning and thunder crackled and choppy surf slapped against piers and seawalls.

The storm was 225 miles (360 kilometers) south-southeast of Puerto Rico, early Thursday, with maximum sustained winds near 40 mph (65 kph). Isaac was moving west near 13 mph (21 kph), the U.S. National Hurricane Center said.

At the bar of the Fort Young Hotel in Dominica's coastal capital of Roseau, a few tourists and locals drank and chatted over the sound of white-crested waves outside.

"The skies were very black and cloudy most of the day, but it's been pretty quiet so far. Some rain, very little wind," bartender Raymond Reynolds said Wednesday at the 71-room hotel on the jagged, densely forested island. "We've been through this before."

In the foothills of Dominica's Morne Aux Diables volcano, Tess Hunneybell, owner of Manico River Eco Resort, said most of Wednesday was "weirdly quiet" after she and others wrapped the resort's signature treehouses in tarpaulin and nailed shut louvre doors.

In the U.S. Virgin Islands, along the harbor in St. Croix's historic town of Christiansted, piers normally lined with pleasure boats were empty Wednesday. Business owners stacked sandbags around the doorways of pastel-colored buildings.

Schools and government offices in St. Croix were ordered to remain closed Thursday. St. Kitts had announced similar closures Wednesday.

With the storm expected to pass just south of Puerto Rico on Thursday, Gov. Luis Fortuno declared a state of emergency for the U.S. territory and activated the National Guard. He also canceled classes and closed government agencies. Federal officials closed the popular San Felipe del Morro castle in Old San Juan.

Authorities in Puerto Rico reported that a 75-year-old woman died Wednesday in the northern city of Bayamon when she fell from a second-floor balcony while filling a barrel with water in preparation for the storm.

The U.S. Coast Guard closed all ports in Puerto Rico and the U.S. Virgin Islands to incoming commercial ships and warned that all commercial vessels bigger than 200 gross tons must leave or obtain permission to remain in port.

Downpours drenched the French island of Guadeloupe on Wednesday, said local chief meteorologist Norbert Aouizerats. Officials warned of swollen rivers and flooding in Martinique, where authorities urged people in low-lying areas to evacuate.

In the Dominican Republic, authorities banned boats from entering its waters and warned of heavy rains from Thursday through Saturday.

Liat airline and American Eagle canceled flights to islands including Dominica, the U.S. Virgin Islands, Guadeloupe and Martinique.

On the island of Vieques, just east of Puerto Rico, people prepared for the government to temporarily shut off power.

Glenn Curry, an owner of Bananas Guesthouse, said he closed the restaurant and would move guests to a higher floor.

"I don't think this is going to be a major storm, but it's going to be noisy and unpleasant for a few hours," he said.

In the meantime, another tropical depression was moving across the Atlantic. The depression had maximum sustained winds near 35 mph (55 kph) early Thursday and was expected to become a tropical storm later Thursday or on Friday. The depression was centered about 1,110 miles (1,790 kilometers) west of the Cape Verde Islands and moving west-northwest near 16 mph (26 kph).